All airlines face these types of operational decisions and how they are resolved involves a number factors. If they cancel JFK-LHR, that leaves an airplane and crew out of position. They must either cancel back to the airplane/crew or move them. In this case, BA made the decision to move them. Likely there was no easy way to cancel back and balance the schedule, particularly if other flights are full. It can also be particularly difficult to find crews to move around and reschedule, especially toward the end of the month.